1. Field of the Invention
The present invention relates to a printed wiring board and electronic equipment capable of solving a problem of interference which takes place within the electronic equipment, in which electromagnetic waves generated by the electronic equipment affect the performance of the electronic equipment itself.
2. Description of the Related Art
In recent years, digital peripheral devices such as a digital camera or a printer are widespread, and personal computers or peripheral devices are connected to each other. Under such circumstances, a device, such as a digital camera and a printer, which has a built-in radio portion and enables connection to the peripheral device through a wireless interface is on the rise.
FIG. 10 is a schematic diagram for explaining a structure of a digital camera having a radio portion incorporated therein. A printed wiring board 140 used for a digital camera is generally formed of two blocks including a camera portion 141 and a radio portion 142. The camera portion 141 constitutes a baseband block, which is driven by a baseband signal of a frequency equal to or less than a sampling frequency necessary for subjecting an image to digital signal processing. The sampling frequency is twice or more as high as the frequency band of the image. For example, it is known as a sampling theorem to use a sampling clock of 16 MHz or higher with respect to a baseband signal having a frequency band of 8 MHz. The radio portion 142 constitutes a high-frequency block which is driven in a frequency band which is used in using a general radio LAN. In general, a frequency band of 2.4 to 2.5 GHz is used.
The camera portion 141 operates based on a clock signal 143, and processing and operation are performed based on the clock signal 143. An image taken by a CCD 145 is sent to a CPU portion 144 through a CCD interface 146 where the image is subjected to digital processing. The processed image is stored in an internal memory (not shown), or in a removable memory card or the like (not shown). An LCD 147, which displays a subject on a monitor screen and indicates various information on the subject, displays the image taken by the CPU portion 144 and various setting information, through an LCD controller 148. Further, a modulation/demodulation processing portion 149 is connected to the CPU portion 144, and signals from the CPU portion 144 are converted into a frequency band used for radio communication. The signals which are converted into a radio frequency band are connected to the radio portion 142 through an interface 153.
The radio portion 142 amplifies high-frequency signals, which are transmitted from the modulation/demodulation processing portion 149 through the interface 153, through a power amplifier 150 for transmission, and transmits the amplified signals as radio signals from an antenna 152. Also, the radio signals received through the antenna 152 are amplified through a low noise amplifier 151 for reception, and transmitted to the modulation/demodulation processing portion 149 through the interface 153.
In the case of a digital camera structured as described above, the camera portion 141 operates based on a high-speed clock signal having a large amplitude. Accordingly, a noise component including many higher harmonic waves is mixed in a power source and a ground of the camera portion 141. Each IC of the camera portion 141 uses digital signals, and each portion operates in synchronization with the clock. For this reason, such noise component leads to a malfunction of each IC. Also, those high-frequency signals are radiated as a radiation noise into space by a conductor such as a ground, which also results in a factor that causes a malfunction.
On the other hand, signals which are exchanged at the interface 153 in a radio frequency band and signals received by the antenna 152 are analog signals which are extremely small in amplitude. For this reason, when the above-mentioned noise component is mixed in the power amplifier 150 of the radio portion 142, a distortion component is generated in an output of the power amplifier 150 due to the nonlinearity of the input/output characteristic of the power amplifier 150. In particular, when an odd-order distortion is generated in the vicinity of the transmission frequency, a spurious noise is transmitted. Further, when the noise component is mixed in the low noise amplifier 151, a spurious noise which is outside of the desired frequency band is received. Also, when the noise component large in amplitude is input to the low noise amplifier 151, the low noise amplifier is saturated, leading to deterioration in reception sensitivity.
For those reasons as described above, in a printed wiring board which is provided with both a baseband block such as the camera portion 141 and a high-frequency block such as the radio portion 142, it is necessary to prevent the two blocks from being coupled to each other in space. In view of the above, there has been proposed to provide a shield case to the high-frequency portion or to separate the power sources and the grounds of the two blocks.
Also, Japanese Patent Application Laid-Open No. 2004-264028 has proposed a technology of preventing reception sensitivity from being deteriorated by a clock signal or the like generated within a radio device.
As described above, when the high-frequency block is covered with the shield case, it is possible to efficiently confine, in particular, a high-level electric field radiated from the power amplifier, to thereby suppress the electric field from interfering with the baseband portion. Also, it is possible to prevent a situation where a clock signal radiated from the baseband portion and higher harmonic waves thereof hinder the reception of faint signals in the low noise amplifier. In other words, the shield case functions effectively with respect to the coupling in space.
Further, with the structure in which the power sources and the grounds of the baseband block and the high-frequency block are electrically separated from each other, a fluctuation occurring in the power source or in the ground of one of the blocks is not transmitted to the other block, to thereby reduce interference occurring through the conductor.
However, according to the above-mentioned interference prevention method, the grounds of the blocks are disconnected from each other, and a return path cannot be formed on the printed wiring board for a signal transmitted through the interface.
FIG. 11 is a schematic diagram illustrating a printed wiring board 131 in which grounds of two blocks are separated from each other. On a top surface of the printed wiring board 131, there are provided a baseband portion 132 and a high-frequency portion 133. The base band portion 132 and the high-frequency portion 133 are connected to each other through a transmission line 136 which functions as an interface. Meanwhile, an undersurface of the printed wiring board 131 is provided with a ground plane 134 and a ground plane 135, the ground plane 134 and the ground plane 135 being separated from each other. The ground plane 134 includes a power source system of the baseband portion 132, and the ground plane 135 includes a power source system of the high-frequency portion 133.
With the above-mentioned structure, the baseband portion 132 is provided close to the ground plane 134, and the baseband portion 133 is provided close to the ground plane 135. In this manner, the closed signal circuits in each of the blocks function satisfactorily. Also, the grounds of the blocks are separated from each other, and therefore interference through the power source and the ground can be reduced. However, the transmission line 136 connecting the blocks to each other does not have a suitable ground plane corresponding thereto, and a return path cannot be formed via a minimal route. For this reason, signals that should be transmitted through the transmission line 136 may be extremely deteriorated in quality, or the signals may not be transmitted in the worst case.
In other words, in the printed wiring board 131 incorporated in electronic equipment, the grounds of the blocks are connected to the frame ground 137 which is generally formed of a casing or the like, but the grounds do not form a satisfactory return path with respect to the above-mentioned interface. The above-mentioned two blocks are connected to the power source of the electronic device to form a long path, which does not serve as an effective return path, either.
Japanese Patent Application Laid-Open No. 2004-264028 proposes a technology of preventing reception sensitivity from being deteriorated by a clock signal or the like generated within a radio device. However, this technology removes higher harmonic waves of the clock falling into a receiving frequency band, and is not capable of dealing with an interference secondarily occurring in the radio portion. Also, it is impossible to prevent a spurious noise secondarily generated in the transmitting portion.